1. Field of the Invention
The present invention relates to the field of refrigeration and air conditioning. More particularly, the invention relates to thermal energy storage apparatus and methods for use in refrigeration and air conditioning systems that incorporate holdover plates.
2. Background of the Invention
Refrigeration and air conditioning systems frequently utilize structures, often called holdover plates, to store thermal energy for later use. Holdover plates generally comprise a metal vessel having serpentine tubing coiled throughout and a thermal energy storage material disposed within the vessel and surrounding the tubing. The tubing serves as the heat exchanger for the associated refrigeration circuit. The thermal energy storage media used are preferably high heat of fusion eutectic mixtures, that is, a mixture of two or more substances that in combination have a single melting/freezing point with large amounts of heat absorbed/evolved during melting/freezing.
Holdover plate refrigeration systems operate by first freezing the thermal energy storage media by use of a powered refrigeration system. Subsequently, the isothermal heat absorption by the thermal energy storage media upon melting provides extended period holdover refrigeration to maintain the system at the desired temperature. The capacity (i.e., holdover period/heat load product) of the holdover system to maintain the system at a specified temperature is a function, in part, of the heat of fusion of the thermal energy storage medium. Other things being equal, materials having a higher heat of fusion can maintain the specified system temperature for a longer period.
With the exception of two properties, brine solutions are ideal for use as thermal energy storage media; brines are typically low cost, non-toxic, have a high heat of fusion (&gt;50 cal/gram), and have a wide range of eutectic temperatures of interest for refrigeration and air conditioning. The two properties that have limited commercial application of brine solutions in holdover refrigeration systems are (1) they are corrosive and (2) they expand upon freezing. Most metals, and almost all combinations of metals, corrode in brine solutions. This corrosion problem severely limits the material choices for holdover plate construction. For example, copper and aluminum, which are excellent thermal conductors, cannot be used.
Aqueous solutions expand on freezing. This property has limited holdover plate designs to those that can withstand the crushing forces exerted by the solutions during freezing. Because expansion on freezing is inherent to aqueous eutectics, brine solutions are not compatible with efficient fin-and-tube heat exchanger systems in which thin walls on the tubing carrying the refrigerant, and thin fins that extend from the tubing into the surrounding thermal energy storage medium, provide efficient heat transfer between the storage solution and the refrigeration cycle. Such delicate fin and tube structures would be crushed by the expansion of the thermal energy storage solution upon freezing.
Previous holdover plate systems incorporating aqueous eutectics relied on heavy-walled tubing, without fins, run through the storage solution to effect the heat transfer between the thermal storage media and the refrigerant circulated through the tubing. This is an inefficient means of heat transfer between the refrigeration circuit and the thermal energy storage medium of the holdover system. The low surface area and thick walls of the tubing in contact with the thermal storage medium impede heat transfer. Additionally, freezing initiates around the refrigeration cycle tubing, requiring that subsequent heat transfer is not only through the thick tube walls but also through the frozen solution surrounding the refrigeration cycle tubing. Freezing on the tubes degrades efficiency by both impeding conduction (the frozen thermal energy storage medium is a poor heat conductor) and eliminating convective heat transfer from the refrigeration tubes to the unfrozen energy storage medium.
The present inventor currently holds a patent (U.S. Pat. No. 4,719,028) on a class of high heat of fusion thermal energy storage solutions that are non-corrosive and do not expand upon freezing. However, these patented solutions are potentially toxic, and are relatively expensive, unlike low cost and non-toxic brine solutions.
Several prior art patents disclose structures and methods for coating heat storage materials to form pellets. These pellets can be incorporated into plaster board and masonry walls of buildings for heat storage. (See Hatfield, U.S. Pat. No. 4,708,812; Griffen, U.S. Pat. Nos. 4,587,279 and 4,617,332; Salyer, U.S. Pat. Nos. 4,908,116, 4,711,813, and 4,797,160; and Chen, U.S. Pat. No. 4,504,402). Another related patent is Driscoll, U.S. Pat. No. 4,325,230, which describes a means of encapsulating a thermal storage solution for use in place of ice cubes. The encapsulation prevents the problem of melting ice diluting drinks. None of the above-mentioned patents pertain to refrigeration or air conditioning systems, and none relate to the problem of corrosiveness or expansion on freezing of thermal energy storage media.